A Self-Supported Stainless Steel Nanocone Array Coated with a Layer

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A Self-Supported Stainless Steel Nanocone Array Coated with a Layer of Ni-Fe Oxides/(Oxy)hydroxides as Highly Active and Robust Electrode for Water Oxidation Junyu Shen, Mei Wang, Liang Zhao, Jian Jiang, Hong Liu, and Jinxuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00498 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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ACS Applied Materials & Interfaces

A Self-Supported Stainless Steel Nanocone Array Coated with a Layer of Ni-Fe Oxides/(Oxy)hydroxides as Highly Active and Robust Electrode for Water Oxidation Junyu Shen, Mei Wang,* Liang Zhao, Jian Jiang, Hong Liu, and Jinxuan Liu State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, China ABSTRACT: Highly efficient, robust, and cheap water oxidation electrodes are of great significance for large-scale production of hydrogen by electrolysis of water. Here, a self-supported stainless steel (SS) nanocone array coated with a layer of nanoparticulate Ni-Fe oxides/(oxy)hydroxides was fabricated by a facile, low-cost, and easily scalable two-step process. The construction of a nanocone array on the surface of AISI 304 SS plate by acid corrosion greatly enlarged the specific surface area of the substrate, and the subsequent formation of a layer of Ni-Fe oxides/(oxy)hydroxides featuring the NiFe2O4 spinel phase on the nanocone surface by electrodeposition of [Ni(bpy)3]2+ significantly enhanced the intrinsic activity and the stability of the SS-based electrode. The as-prepared electrode demonstrated superior activity for oxygen evolution reaction (OER) in 1 M KOH, with 232 and 280 mV overpotentials to achieve 10 and 100 mA cmgeo−2 current densities, respectively. The high activity of the electrode was maintained over 340 h of chronopotentiometric test at 20 mA cmgeo−2, and the electrode also showed good stability over 100 h of electrolysis at high current density (200 mA cm−2). More important for practical application, the used SS-based electrode can be easily regenerated with original OER activity. The superior activity of this SS-based electrode stems from synergistic combination of high conductivity of the SS substrate, a large electrochemically active surface area of the nanocone array, and a uniformly coated nanoparticulate Ni-Fe oxide/(oxy)hydroxide layer with an optimal Ni/Fe ratio.

KEYWORDS: electrolysis, Ni-Fe oxide/(oxy)hydroxide, oxygen evolution, stainless steel, water oxidation 1

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INTRODUCTION Developing clean, affordable, and sustainable energy systems to replace those based on fossil fuels is one of the most important scientific issues today when energy shortage and environmental pollution have become increasingly serious problems. The promising approaches to relieving the stress from these knotty problems in the modern society are the reduction of water to hydrogen and the conversion of carbon dioxide to energy-dense liquid or gaseous fuels, by employing solar energy or electric power generated from renewable energy sources. For production of hydrogen from electrochemical water splitting, the oxygen evolution reaction (OER), as an efficiency-limiting half reaction, plays a key role in the construction of energy-efficient and cost-effective devices and systems. Therefore, highly efficient and robust OER electrocatalysts are of great significance to facilitate the intrinsically sluggish kinetics of water oxidation by reducing the large overpotential of OER. In recent years, tremendous earth abundant metal-based electrocatalysts, especially Fe, Co, and Ni oxides/(oxy)hydroxides1−18 and layered double hydroxides (LDHs),19−30 have been extensively studied for OER. Some of these electrocatalysts display competent or even higher OER activity and better stability than IrO2 and RuO2 in strong basic electrolytes. Several state-of-the-art earth abundant catalysts modified on flat substrates such as glassy carbon, copper plate, and graphite exhibited 10 mA cmgeo−2 (geometric area) catalytic current density at overpotentials lower than 300 mV,9‒12,30‒37 and only a few Ni, Co, and Fe-based catalysts loaded on metal foams and carbon fiber paper (CFP) were reported to display 10 mA cmgeo−2 current density at overpotentials lower than 200 mV.9,29,38‒43 The research in this field has been progressing rapidly and the benchmarking performance of OER electrocatalysts is constantly refreshed. Nevertheless, the lack of highly active, robust, and cheap electrodes for OER is still an obstacle for the large-scale production of hydrogen by electrolysis of water. Stainless steel (SS) is an ideal substrate for OER given its merits of good mechanical

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robustness and high electrical conductivity. Besides, it is the cheapest and most ubiquitous material being employed as electrode substrates for water oxidation (Table S1 in the Supporting Information). Reports on SS-based OER electrodes have been emerging continuously in recent years.44‒58 The best performance of reported Fe-based cheap SS plate electrodes is to achieve 10 mA cmgeo−2 current density at about 210 mV overpotential in 1 M KOH.52 Additionally, the N-doped and N,P-doped surface-etched AISI 304 SS meshes exhibit 278 mV at 10 mA cmgeo−2.54 The surface modification strategies for reported SS-based OER electrodes are i) treatment of SS substrates with Cl2 gas, ii) electrochemical corrosion of SS in caustic alkali solution and normally at high current density, iii) hydrothermal corrosion of SS in NH3/H2O or in KOH/NaOCl solution at high temperature, and iv) sulfurization, nitridation or phosphorization of SS substrates at 400‒600 °C. Although these protocols can apparently enhance the catalytic activity of SS-based electrodes for OER, the operation procedures are involved in harmful gases, very caustic solutions, high temperature, enduring pressure, and highly energy-consuming processes. From a large-scale commercial application point of view, not only the OER and hydrogen evolution reaction (HER) electrocatalysts are desired to be highly active, robust and cheap, but also the processes for fabrication of OER and HER electrodes are expected to be low-cost, easy to operate, readily scalable, low energy-consuming, and environment benign.59 Herein, we report a simple and mild process for the surface modification of cheap austenitic AISI 304 SS plate through electrochemical acid corrosion followed by deposition of nickel (oxy)hydroxide from the nickel tris(bipyridine) complex. The processing strategy was first creating the SS substrate with a large specific surface area by forming a self-supported nanocone (NC) array on its surface, and then enriching the Ni (oxy)hydroxide content on the surface of the NC array to the optimal Ni/Fe ratio. The as-prepared SS-based electrode, denoted as Ni-Fe/SS-NC, required very small overpotentials of 241 and 280 mV to achieve 20 and 100 mA

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cmgeo−2 current density, respectively, in 1 M KOH. More important for practical application, the SS-based electrode displayed good stability over 100 h of electrolysis at high current density of 200 mA cm−2, and the used SS-based electrode can be easily regenerated with original OER activity. This SS-based electrode could hold promising for large-scale application in commercial water splitting electrolyzers given its superior activity at low overpotential, good stability, facile fabrication process, suitability for scale-up and recycling, as well as very low-cost.

EXPERIMENTAL SECTION SS-NC Electrode Preparation. An AISI 304 SS plate was polished with 300, 600 and 1000 mesh abrasive paper and washed with ethanol, acetone and deionized water successively, and then used as the working electrode (1 cm2). The solution containing NaH2PO4 (0.1 M) and NaBF4 (0.1 M) was adjusted with 40% HBF4 to pH 2 and used as supporting electrolyte. The electrochemical acid corrosion was carried out at an applied potential of 1.85 V versus NHE for 1 h at room temperature, with a Ag/AgCl as the reference electrode and a carbon paper (2 cm2) as the counter electrode. After electrolysis, a light yellow film appeared on the surface of the SS plate, and the as-prepared SS-NC plate was rinsed with deionized water for several times. Ni-Fe/SS-NC

Electrode

Preparation.

The

electrochemical

deposition

of

nickel

(oxy)hydroxide was carried out at 0.7 V versus NHE, with SS-NC (1 cm2) as the working electrode, a Hg/HgO and Pt foil (2 cm2) as the reference and counter electrode, respectively, in 1 M KOH solution (30 mL) containing [Ni(bpy)3](BF4)2 (1.0 mM) for 10 h at room temperature. A casing tube with a porous glass frit bottom was used for the Pt foil to prevent nickel complex from depositing at the counter electrode. During the deposition process, the current density increased dramatically and stabilized at 45 mA cm−2. After electrolysis, the as-prepared dark brown Ni-Fe/SS-NC plate was rinsed with deionized water for several times. Fe(Ni)/SS-PM (Porous Material) Electrode Preparation. The electrochemical base

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corrosion was carried out at 0.72 V versus NHE in 1 M KOH for 4 h using the same working, reference and counter electrodes as those for the fabrication of Ni-Fe/SS-NC. Following workup was identical with that used for Ni-Fe/SS-NC. Materials Characterization. Scanning electron microscopy (SEM) images were taken on a NOVA NanoSEM 450 or HITACHI UHR FE-SEM SU8220 instrument with an acceleration voltage of 3 kV. SEM energy-dispersive X-ray (EDX) spectra were obtained at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM) images, TEM-EDX spectra, EDX element mapping images, and selected area electron diffraction (SAED) patterns were collected on a FEI Tecnai G2 F30 S-TWIN transmission electron microscope with an acceleration voltage of 300 kV. X-ray photoelectron spectra (XPS) measurements were taken on a Thermo VG ESCALAB250 surface analysis system using a monochromatized Al Kα small-spot source and a 500 µm concentric hemispherical energy analyzer. X-ray diffraction (XRD) patterns were measured with a Panalytical Empyrean using Cu Kα radiation (λ = 1.5405 Å); the data were collected in Bragg-Brettano mode in the 2θ range from 5° to 85° at a scan rate of 5° min−1. Raman spectra were recorded on a Thermo Scientific DXR Raman spectrometer with 514 nm excitation wavelength and 1.0 mW laser power level. Inductively coupled plasma optical emissions spectrometry (ICP-OES) analyses were made on a PerkinElmer 2000 DV instrument. Electrochemical Measurements. All electrochemical tests were performed on a CHI660E workstation (CH instruments Inc.), with a Hg/HgO electrode as the reference electrode and a Pt foil (2 cm2) as the counter electrode. All electrochemical measurements were carried out in 1 M KOH, and the experimentally measured potentials versus Hg/HgO were converted to the potentials versus reversible hydrogen electrode (RHE) by the equation: E(Hg/HgO) + 0.098 + 0.059pH. Cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) were measured using the as-prepared SS-based electrodes (geometric area of front and back side (0.4 cm2 × 2) + lateral area (0.08 cm2 × 2) + bottom area (0.05 cm2) ≈ 1 cm2) as working electrodes,

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and the polarization curves were corrected with 85% iR compensation, while the long-term chrononopotentiometric experiments were carried out using Ni-Fe/SS-NC as the working electrode

without

iR

compensation.

Electrochemical

impedance

spectroscopy

(EIS)

measurements were carried out at an applied potential of 1.5 V with frequency ranging from 1 Hz to 100 kHz. The double layer capacitance charging curves were measured with sweeping between 0.977 to 1.027 V at varying scan rates (10−100 mV s−1), and the capacitive currents (∆j = |jcharge – jdischarge|) were measured at 1.00 V.

RESULTS AND DISCUSSION Fabrication of Ni-Fe/SS-NC and Related Reference Electrodes. The two-step process, i.e., acid corrosion and nickel (oxy)hydroxide deposition, for fabricating the Ni-Fe/SS-NC electrode is schematically shown in Figure 1 (The detailed preparation protocols are given in the Supporting Information, Figure S1). The purpose of the first step was to tailor the surface morphology of the SS substrate and thereby to enlarge its specific surface area to support more active sites on a given electrode. The AISI 304 SS plate was electrochemically corroded in NaH2PO4 (0.1 M)/NaBF4 (0.1 M) solution at pH 2 (adjusting with 40% HBF4) for 1 h at 1.85 V versus normal hydrogen electrode (NHE) (Figures S2a, S3 and S4 in the Supporting Information). Appealingly, a NC array was formed on the SS plate surface, which was evidenced by SEM images (Figure 1c; Figure S5a,b in the Supporting Information). SEM and TEM images showed that the NCs were 3‒6 µm long with their tips crooked like squid tentacles (Figure S5b,c in the Supporting Information). It was noticed that without acid or when HBF4 acid was replaced by HCl, H2SO4, HNO3, and H3PO4 the similar NC array could not be formed under otherwise identical conditions. In addition, the pH of electrolyte for the SS corrosion was also an important factor to affect the morphology of SS surface. The NCs could not be formed at pH 4 (Figure S6 in the Supporting Information), while the NCs completely dropped off from the

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surface at pH 1 after 1 h of acid corrosion. The best NC array was obtained in the NaH2PO4/NaBF4/HBF4 solution at pH 2 in 1 h. The SS plate obtained from the first step is denoted as SS-NC. A number of studies have revealed that the existence of 10%‒50% Fe in Ni oxide/(oxy)hydroxide substantially improved the OER activity,7,26,60‒62 while the presence of Cr oxide on the surface of SS led to significant decrease in the OER activity.44,47,50,52 Therefore, the purpose of the second processing step was to enrich the content of Ni (oxy)hydroxide on the electrode top-surface, meanwhile to deplete Cr at the surface of self-supported SS NCs, so as to improve the intrinsic activity of SS-based electrodes for OER. Because Ni(OH)2 was precipitated immediately upon addition of nickel salts to 1 M KOH solution, the electrodeposition of Ni (oxy)hydroxide was made with [Ni(bpy)3](BF4)2 as a precursor in terms of its good solubility in basic solution and facile preparation process. The electrodeposition was carried out at 0.7 V versus NHE in 1 M KOH solution containing [Ni(bpy)3](BF4)2 (0.33‒1.67 mM) for 10 h with SS-NC as a working electrode (Figures S2b and S3 in the Supporting Information). The two-step fabrication procedure for OER electrodes using cheap reagents and materials with facile manipulation under mild conditions is easily scalable at low cost. To be specific, the cost for lab scale fabrication of Ni-Fe/SS-NC (1 cm2) was estimated to be 0.13 $ based on the prices of reagents and materials in Chinese market (Table S2 in the Supporting Information).

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Figure 1. (a) Schematic fabrication process of Ni-Fe/SS-NC through acid corrosion and Ni (oxy)hydroxide deposition. SEM images of (b) polished AISI 304 SS plate, (c) SS-NC, and (d) Ni-Fe/SS-NC. (e) TEM image of NCs peeled off from the Ni-Fe/SS-NC electrode.

To make comparison, some reference electrodes were fabricated. In the second step, when SS-NC was electrolyzed in 1 M KOH in the absence of [Ni(bpy)3](BF4)2, the NC array on its surface was collapsed by base corrosion, forming porous material and some small pits on the electrode surface (Figures S7 and S8a in the Supporting Information). This observation implies that the deposited Ni (oxy)hydroxide layer played a role of protecting the NC surface from base corrosion. The SS-based reference electrode fabricated in the absence of nickel complex is denoted as Fe(Ni)/SS-PM. SEM-EDX spectrum and elemental mapping images (Figures S8 and S9b in the Supporting Information) showed that the porous material at the Fe(Ni)/SS-PM electrode surface was composed of uniformly distributed Fe, Ni, and O, but had negligible Cr element, while the smooth surface of pits contained Fe, Ni, and Cr, but had no O element. In addition, Ni (oxy)hydroxide was also electrochemically deposited from [Ni(bpy)3](BF4)2 in 1 M KOH on the flat SS plate, which had not been pretreated by acid corrosion. This electrode is denoted as Ni-Fe/SS (Figure S10 in the Supporting Information). 8


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On the basis of SEM image of the as-prepared Ni-Fe/SS-NC plate (Figure 1d), the NC array remained on the SS plate surface after deposition of Ni (oxy)hydroxide in basic solution. TEM image (Figure 1e) of NCs peeled off from the surface of Ni-Fe/SS-NC showed that a thin layer of nanoparticles decorated around the SS NCs. Moreover, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 2a), EDX spectrum (Figure S9c in the Supporting Information), and elemental mapping images of the NCs revealed a uniform distribution of Fe, Ni, and O elements at the NC surface. The Raman spectrum (Figure 2c) of Ni-Fe/SS-NC revealed that the nanomaterial on the electrode surface was composed of Ni-Fe oxides/(oxy)hydroxides. The pattern and locations of bands appearing in the region of 100−900 cm−1 matched well with those of the NiFe2O4 spinel phase,60 while the bands of other Ni-Fe oxides/(oxy)hydroxides were possibly buried in the broad bands at 478, 551, and 678 cm−1.26,63 The weak band centered at 1302 cm−1 might be originated from FeOOH.64 In the powder X-ray diffraction (PXRD) pattern (Figure 2d) of Ni-Fe/SS-NC, the two humps centered at 2θ = 35.1° and 62.5° were consistent with those from the NiFe2O4 lattice (JCPDS No. 86-2267);60 the weak and broad peaks indicated the existence of a short-range order in the NiFe2O4 phase. Furthermore, the SAED (Figure 2b, inset) also indicated the existence of polycrystalline NiFe2O4 phase in the material on the surface of Ni-Fe/SS-NC.

The

Raman

spectrum

(Figure

2c)

of

Fe(Ni)/SS-PM

showed

Fe

oxide/(oxy)hydroxide is the dominant ingredient on its surface. The bands at 214, 275, 391, 586, 671, and 1289 cm−1 could be attributed to Fe2O3, Fe3O4, and FeOOH.26,64 Additionally, the Raman spectrum (Figure 2c), together with PXRD (Figure 2d) and SAED patterns (inset of Figure S7c in the Supporting Information) suggested the existence of the poorly crystalized NiFe2O4 spinel phase on the surface of Fe(Ni)/SS-PM, although in a low content. As there were no other detectable diffraction peaks in the PXRD patterns of Ni-Fe/SS-NC and Fe(Ni)/SS-PM, the Ni-Fe oxides/(oxy)hydroxides, except for NiFe2O4, should exist in amorphous nature at the

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electrode surface, which was further demonstrated by high magnification TEM images (Figure 2b; Figure S7c in the Supporting Information). In contrast, the Raman spectrum and XRD patterns of Ni-Fe/SS suggested the existence of Ni-Fe oxides/(oxy)hydroxides as the major gradients on the electrode surface.

Figure 2. (a) HAADF-STEM and EDX elemental mapping images of Fe, Ni and O for the top part of the Ni-Fe/SS-NC. (b) High magnification TEM image of Ni-Fe/SS-NC (Inset: SAED pattern of the NC material from Ni-Fe/SS-NC). (c) Raman spectra and (d) PXRD patterns of the powder samples peeled off from Fe(Ni)/SS-PM and Ni-Fe/SS-NC, as well as XRD patterns of Ni-Fe/SS and AISI 304 SS plates.

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XPS (Figure S11 in the Supporting Information) indicated that the predominant elements on the surface of SS-NC were Fe, Ni, Cr, P, B, and O, which was consistent with the result of EDX analysis (Figure S9a in the Supporting Information). The P and B elements came from phosphate and fluoroborate ions that were strongly adsorbed on the electrode surface. In contrast, the Cr 2p, P 2p and B 2p peaks were not detected in XPS and EDX spectra (Figure S9b,c in the Supporting Information) of Fe(Ni)/SS-PM and Ni-Fe/SS-NC. ICP-OES analyses (Table S3 in the Supporting Information) demonstrated that these elements were dissolved in electrolyte when SS-NC was treated in 1 M KOH solution. The Ni/Fe atomic ratio on the surface of Ni-Fe/SS-NC was 1.60:1 (Table S4 in the Supporting Information), which was 32 times higher than that of Fe(Ni)/SS-PM (Ni/Fe = 0.05:1). As expected, the electrodeposition of [Ni(bpy)3]2+ significantly increased the Ni(OH)2/NiOOH contents on the electrode surface, which would benefit the catalytic performance of SS-based electrodes for OER. Moreover, the composition depth profile of XPS (Figure S12 in the Supporting Information) showed that the Ni/Fe atomic ratio decreased from 1.7:1 to 1.04:1 with increase of the etching depth from 0 to 20 nm. In the Fe 2p3/2 region (Figure 3a), the peak at 707 eV for Fe0 observed in XPS of the pristine AISI 304 SS was not detected for SS-NC, Fe(Ni)/SS-PM, Ni-Fe/SS, and Ni-Fe/SS-NC.47,49−51,54 The broad peak centered at 711.9 eV was attributed to the Fe3+ salts with tetrafluoroborate and phosphate counter ions at the surface of SS-NC, while the peaks appearing at 711.1‒711.3 eV for Ni-Fe/SS, Fe(Ni)/SS-PM, and Ni-Fe/SS-NC were ascribed to the Fe3+ 2p3/2 of Fe (oxy)hydroxide phase.18,47,50,64,65 Figure 3b shows that there is a very small amount of Ni2+ on the surfaces of SS-NC and Fe(Ni)/SS-PM; in contrast, the Ni-Fe/SS and Ni-Fe/SS-NC display apparent peaks centered at 855.4‒855.6 eV with a satellite peak at 861.2‒861.4 eV, which can be assigned to Ni 2p3/2 of a Ni(OH)2 or NiOOH phase.47‒50,52 Additionally, the fitted small peak at 854.4 eV is ascribed to NiO. The Cr 2p3/2 spectra (Figure 3c) demonstrated that Cr2O3 (576.6

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eV) on the surface of an untreated SS plate was converted to Cr6+ oxides (centered at 579.1 eV) on the SS-NC plate surface,65 while there was no Cr species on the surfaces of Fe(Ni)/SS-PM, Ni-Fe/SS and Ni-Fe/SS-NC plates. As for O 1s spectra of the treated SS plates, there were three fitted peaks located at about 529.8, 531.1, and 531.7 eV (Figure 3d), which are assigned to O2−, OH−, and adsorbed H2O, respectively, at the electrode surface.50,66

Figure 3. XP spectra of (a) Fe 2p3/2, (b) Ni 2p3/2, (c) Cr 2p3/2, and (d) O 1s for Ni-Fe/SS, SS-NC, Fe(Ni)/SS-PM, and Ni-Fe/SS-NC, as well as AISI 304 SS.

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The formation of NC array on the surface of an AISI 304 SS substrate during acid corrosion is most possibly due to evenly distributed Cr2O3 at the SS surface (Figure 3c and Figure S13 in the Supporting Information), which has a better resistance against acid corrosion compared to iron and nickel oxides.67 The acid corrosion of the parts beneath the Cr2O3-passivated spots would be slowed down, leading to the formation of NC arrays. This speculation is supported by the experimental results that a commercial Ni42Fe58 alloy plate without Cr formed a nanoporous surface instead of NC array under the same conditions employed for treatment of the AISI 304 SS plate (Figure S14a in the Supporting Information), while a Cr20Ni80 alloy plate was corroded to form uniformly distributed microislands on its surface (Figure S14b in the Supporting Information). In the light of the fact that Cr was detected from the surface of SS-NC, but not from the surfaces of Ni-Fe/SS-NC and Fe(Ni)/SS-PM electrodes by EDX and XPS (Figure 3c and Figure S9 in the Supporting Information), it is deduced that Cr element plays a key role in the formation of a NC array morphology on the surface of a SS substrate, but has no contribution to the high OER activity of Ni-Fe/SS-NC and Fe(Ni)/SS-PM electrodes. Electrochemical

Performances

of

Ni-Fe/SS-NC

and

Reference

Electrodes.

Electrochemical properties of Ni-Fe/SS-NC as well as related reference electrodes were evaluated in 1 M KOH solutions using a three-electrode cell with the as-prepared electrode (1 cm2) as working electrode, a Hg/HgO and Pt foil (2 cm2) as the reference and counter electrodes, respectively. The Ni-Fe/SS-NC plate displayed a wave at Ep = 1.43 V (All potentials given in this paper are versus RHE except where otherwise noted.) in LSV (Figure 4a and Figure S15a in the Supporting Information), which is attributed to the oxidation process of Ni(OH)2 to NiOOH.61 To clearly observe the OER onset potential and the overpotential at 10 mA cmgeo−2, LSV of Ni-Fe/SS-NC was scanned from positive to negative direction (Figure S15b in the Supporting Information), which revealed that the onset potential was 1.44 V (ηonset = 210 mV)

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and the overpotential at 10 mA cmgeo−2 was only 232 mV. These OER catalytic data are coincident with those reported for the NiFeOx on carbon fiber paper,68 which is one of the best OER electrocatalysts reported to date. When the applied potential was scanned to the voltage higher than 1.44 V, gas bubbles were released from the surface of the working electrode. The Ni-Fe/SS-NC plate demonstrated very low overpotentials of 241 and 280 mV to attain 20 and 100 mA cmgeo−2 current density, respectively. Among the reported SS plate-based OER electrodes (Table S5 in the Supporting Information), only the one made with an X20CoCrWMo10-9 SS plate displayed an overpotential lower than 250 mV to achieve an current density of 20 mA cmgeo−2;55 the overpotentials necessary to reach 100 mA cmgeo−2 for reported SS plate electrodes are larger than 300 mV.44−51,53,54 To our knowledge, only a few earth-abundant element electrocatalysts have been reported to achieve 20 mA cmgeo−2 current density at overpotential smaller than 250 mV without needing high specific area materials such as metal foams and carbon fiber paper as substrates (Table S6 in the Supporting Information).9,27,31 The performance of Ni-Fe/SS-NC is superior to that of the self-made IrO2/AISI 304 SS plate (IrO2 1.26 mg, the same amount as that of the total NC material on the Ni-Fe/SS-NC plate) and the commercial IrO2-RuO2/Ti electrode for OER (Figure S16), which displayed the current density of 20 mA cmgeo−2 at overpotentials of 302 and 323 mV, respectively, in 1 M KOH (Figure S15c in the Supporting Information).

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Figure 4. (a) OER polarization curves of Ni-Fe/SS-NC, Fe(Ni)/SS-PM, Ni-Fe/SS, AISI 304 SS, and IrO2/AISI 304 SS, IrO2-RuO2/Ti reference electrodes in 1 M KOH at a scan rate of 5 mV s−1 with iR compensation. (b) Tafel slopes. Chronopotentiometric curves of Ni-Fe/SS-NC (c) at 20 mA cm−2 over 340 h of electrolysis and (d) at 200 mA cm−2 over 100 h of electrolysis (without iR compensation).

Figure 4a clearly illustrates that the pristine AISI 304 SS plate does not show any current response until scanning to 1.55 V. The reference electrodes, Fe(Ni)/SS-PM and Ni-Fe/SS, exhibited an OER catalytic activity apparently lower than that of Ni-Fe/SS-NC. It is worth noting that there is no oxidation wave at 1.35‒1.46 V in LSV of the Fe(Ni)/SS-PM electrode, which is consistent with the low Ni content on the electrode surface evidenced by the XPS 15



(Figure 3b) and EDX spectrum (Figure S9b in the Supporting Information). The results from these comparative studies prove that both acid corrosion and Ni (oxy)hydroxide deposition processes are indispensable steps to prepare highly active AISI 304 SS-based electrodes for OER. Subsequently, the influence of the concentration of [Ni(bpy)3](BF4)2 in the electrodeposition solution on the activity of Ni-Fe/SS-NC was studied. It was found that as the concentration of [Ni(bpy)3](BF4)2 increased from 0.33 to 1.0 mM the area of Ni(OH)2/NiOOH oxidation wave was apparently augmented (Figure S17 in the Supporting Information), indicating a continuous increase of Ni content on the surface of as-prepared electrodes.2 It is rational that the catalytic current is substantially enhanced with the increase of Ni content on the electrode surface. Further increase of the concentration of [Ni(bpy)3](BF4)2 to 1.67 mM did not bring about notable change in the Ni(OH)2/NiOOH wave and catalytic current. On the basis of XPS analysis (Figure S18 and Table S7 in the Supporting Information), the surface content of Ni notably increased with enhancement of the concentration of [Ni(bpy)3](BF4)2 from 0.33 to 1.0 mM in the electrodeposition solution. These XPS results are in good agreement with the observations from LSVs in Figure S17 in the Supporting Information. The atomic ratio of Ni/Fe went up apparently (from 0.84 to 1.60) as the concentration of [Ni(bpy)3](BF4)2 was increased from 0.33 to 1.0 mM, corresponding to the decrease of the composition of Fe from 54.4 atomic% to 38.5 atomic% on the electrode surface; with further increase of the concentration of [Ni(bpy)3](BF4)2 to 1.67 mM, only a slight enhancement of Ni content on the electrode surface was detected by XPS. Under the optimized conditions (1.0 mM [Ni(bpy)3](BF4)2), the loading amount of extraneous Ni as Ni oxide/(oxy)hydroxide is about 0.632 mg cm−2. The influence of Ni/Fe ratio on the OER activity of Ni-Fe oxide/(oxy)hydroxide films has been studied in alkaline electrolytes by many groups.26,30,60,62,69 The discrepant optimum contents of Fe are reported, ranging from 10 mol% to 50 mol%. The results reported by Bell and

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co-workers showed that the amorphous Ni-Fe oxide/(oxy)hydroxide material with a composition of 40 mol% Fe exhibited the best OER activity.26 Our observations are fully consistent with the literature results. Among a series of the as-prepared Ni-Fe/SS-NC plates with different ratios of Ni to Fe on their surfaces, the electrodes with 34‒39 atomic% of Fe on the surface displayed higher OER activity than those with 46‒54 atomic% of Fe on the electrode surface (Table S7 in the Supporting Information). The Fe(Ni)/SS-PM plate with about 95 atomic% of Fe on its surface exhibited considerably larger overpotential and lower activity than that of Ni-Fe/SS-NC. These results clearly indicate that the optimized atomic ratio of Ni/Fe elements is one of the important factors contributing to the superior catalytic activity of Ni-Fe/SS-NC. In addition to current density and overpotential, another key metric to evaluate an electrode is its Tafel slope. The Ni-Fe/SS-NC plate displayed a Tafel slope (51.1 mV dec−1) smaller than those observed for IrO2/AISI 304 SS (52.8 mV dec−1) and IrO2-RuO2/Ti (77.1 mV dec−1), as well as the reference electrodes Fe(Ni)/SS-PM (58.1 mV dec−1) and Ni-Fe/SS (63.9 mV dec−1) (Figure 4b), while the untreated AISI 304 SS plate exhibited a quite large Tafel slope of 105.8 mV dec−1. The small Tafel slope of Ni-Fe/SS-NC suggests its highly facilitated OER kinetics. The stability of an electrode used for OER is vital to its practical application in commercial water-splitting electrolyzers. The chronopotentiometric experiments revealed that the Ni-Fe/SS-NC plate had perfect stability over 340 h of electrolysis in 1 M KOH, with only about 1% increase in overpotential at a constant current density of 20 mA cmgeo−2 (Figure 4c). More important for the practical use, the Ni-Fe/SS-NC electrode also showed a good stability at high current density of 200 mA cmgeo−2, with merely about 5% increase in overpotential in the end of 100 h of chronopotentiometric electrolysis experiment (Figure 4d). Moreover, the original OER activity of Ni-Fe/SS-NC could be recovered when the used Ni-Fe/SS-NC electrode was re-prepared according to the afore-mentioned two-step process (Figure S19 in the Supporting Information). An Faradaic efficiency of 96.3% was obtained over 3 h of electrolysis of

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Ni-Fe/SS-NC at 1.5 V (Figure S20a in the Supporting Information), while slightly lower Faradaic efficiency was attained for Fe(Ni)/SS-PM (93.5%; Figure S20b in the Supporting Information). The high Faradaic efficiency of Ni-Fe/SS-NC indicates that considerable oxidation of internal metallic substrate beneath the Ni-Fe oxide/(oxy)hydroxide layer could be excluded during electrolysis process. SEM images measured after long-term electrolysis showed that the surface morphology of Ni-Fe/SS-NC did not have apparent change (Figure S21 in the Supporting Information). PXRD patterns of Ni-Fe/SS-NC before and after used for long-term electrolysis were almost identical, indicating that the material on the electrode surface was still in an amorphous state (Figure S22a in the Supporting Information). In addition, Raman spectrum and XPS of the used Ni-Fe/SS-NC plate did not show noticeable change compared to that of the Ni-Fe/SS-NC plate before used for OER (Figures S22b and S23 in the Supporting Information), which suggested that the active ingredient and the oxidation states of Ni and Fe remained predominantly unchanged. Specifically, based on the XPSs measured after the long-term electrolysis experiment, the Ni/Fe atomic ratio on the surface of the used Ni-Fe/SS-NC electrode is estimated to be 1.49:1, which is slightly lower than that (Ni/Fe = 1.60:1) obtained from the XPS of the as-prepared Ni-Fe/SS-NC electrode before used for electrolysis. ICP-OES analysis of the resulting electrolyte after 340 h of electrolysis at the current density of 20 mA cm−2 shows that only 5.9% (0.037 mg) Ni is lost from the Ni-Fe/SS-NC electrode and dissolved in the electrolyte, which is in good agreement with the XPS results. Little loss of Ni (oxy)hydroxide could be the reason for the slight decrease of the OER activity of Ni-Fe/SS-NC during long-term electrolysis. Understanding the Superior OER Performance of Ni-Fe/SS-NC. To understand the superior OER performance of Ni-Fe/SS-NC, we first evaluated the electrical conductivities of different SS-based electrodes by making EIS measurements in 1 M KOH at overpotential of 270 mV. The equivalent circuit fittings of Nyquist plots (Figure 5a) give interface charge-transfer

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resistances (Rct; Table S8 in the Supporting Information) in an increasing order of Ni-Fe/SS-NC (Rct ≈ 0.25 Ω cm2) < Fe(Ni)/SS-PM (0.60 Ω cm2) < Ni-Fe/SS (1.65 Ω cm2). All prepared SS-based electrodes exhibited much better electroconductibility than the pristine AISI 304 SS plate (Rct = 6.53 Ω cm2), which is generally considered as a good conductive material. The extremely low charge-transfer impedance of Ni-Fe/SS-NC is attributed to the following factors: i) the NC array is self-supported on the SS substrate and hence there is no solid surface/surface contact resistance, ii) the inner uncorroded SS substrate beneath the Ni-Fe oxide/(oxy)hydroxide layer ensures the high electron conductivity of Ni-Fe/SS-NC, and iii) the large contact interface between the catalytic film and electrolyte could reduce the charge-transfer resistance. The small Rct value of Ni-Fe/SS-NC is actually one of the crucial factors contributing to the high intrinsic OER activity of the electrode.

Figure 5. (a) Nyquist plots of Ni-Fe/SS-NC and reference electrodes at overpotential of 270 mV in 1 M KOH. (b) Plots of ∆j/2 against scan rate (ν).

In another aspect, after treated with acid corrosion and Ni (oxy)hydroxide deposition the specific surface area of the SS plate was significantly enlarged, which could effectively increase the active sites on the electrode. The NC material peeled off from the surface of the as-prepared

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Ni-Fe/SS-NC plate possessed a Brunauer-Emmett-Teller (BET) surface area of 220.8 m2 g−1 (Figure S24 in the Supporting Information), which was larger than that of the material from Fe(Ni)/SS-PM (182.9 m2 g−1) and about 11.7 times that of the sample from SS-NC (18.9 m2 g−1). The

self-supported

SS

NCs

covered

with

a

layer

of

nanoparticulate

Ni-Fe

oxides/(oxy)hydroxides on the surface of Ni-Fe/SS-NC afford an extremely large contact interface between electrode and electrolyte on a given geometric area, which would benefit the catalytic performance of SS-based electrodes for OER. Moreover, the electrochemically active surface area (ECSA) was estimated by measuring the double layer capacitances (Cdl) of the SS-based electrodes. The Cdl values (Table S8 in the Supporting Information) are equivalent to the linear slopes in Figure 5b, which are derived from the cyclic voltammograms (Figure S25 in the Supporting Information) measured at varying scan rates in a non-Faradaic region. As expected, the Cdl values increase in an order of AISI 304 SS < Ni-Fe/SS < Fe(Ni)/SS-PM < Ni-Fe/SS-NC. The ECSAs for the SS-based electrodes were calculated on the basis of the equation: ECSA = Cdl/Cs, where Cs is the specific capacitance of the material (Here Cs = 0.24 mF cm−2 for the pristine AISI 304 SS plate in 1 M KOH). The Ni-Fe/SS-NC electrode has a much larger ECSA (3.3 cm2) than Fe(Ni)/SS-PM (1.8 cm2) and Ni-Fe/SS (1.4 cm2) electrodes. The large ECSA of the NC array electrode is another important factor contributing to the superior OER performance of Ni-Fe/SS-NC. In order to evaluate the improvement in the intrinsic activity of Ni-Fe/SS-NC, the OER polarization curves of Ni-Fe/SS-NC, Fe(Ni)/SS-PM, and Ni-Fe/SS against applied potential was drawn with j values normalized by ECSA (Figure 6) to exclude the geometric gain in current density. Apparently, the j(ECSA) value of Ni-Fe/SS-NC is much higher than those of Fe(Ni)/SS-PM and Ni-Fe/SS. The results indicate that apart from the large ECSA the improvement of intrinsic activity makes important contribution to the high OER activity of Ni-Fe/SS-NC. Furthermore, we tried to have an insight into the active ingredient that plays a key

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role in improvement of the intrinsic activity of Ni-Fe/SS-NC. For this, the following experimental facts have been noticed. First, the Ni-Fe/SS electrode, with Ni-Fe oxides/(oxy)hydroxides as the dominant ingredients at its surface, displayed a much lower j(ECSA) value than Ni-Fe/SS-NC and Fe(Ni)/SS-PM. Second, Fe(Ni)/SS-PM, with Fe oxide/(oxy)hydroxide as the dominant ingredients on its surface, exhibited a fairly high OER activity, notwithstanding the intrinsic activity of Fe (oxy)hydroxide was proved to be much lower than that of the coexisting Fe-Ni (oxy)hydroxides.28‒31,60,61 Notably, in addition to Fe oxide/(oxy)hydroxide, the surface material of Fe(Ni)/SS-PM also contains a low content of the polycrystalline NiFe2O4 although it is much less than that existing at the surface of Ni-Fe/SS-NC, as evidenced by Raman spectrum, PXRD and SAED patterns. Considering the previous reports on the high activity of the NiFe2O4 spinel phase for electrochemical water oxidation,60 we speculate that the NiFe2O4 spinel phase at the surface of the SS-based electrodes plays a crucial role in improvement of the intrinsic OER activity of Ni-Fe/SS-NC and Fe(Ni)/SS-PM. Besides, NiFe2O4 also shows good corrosion resistance under oxygen evolution conditions,70 which would provide important contribution to the excellent stability of Ni-Fe/SS-NC. Taking account of all above-mentioned experimental evidence, the superior catalytic activity of Ni-Fe/SS-NC is ascribed to the high conductivity and large ECSA of the self-supported SS NC array, the uniformly coated nanoparticulate Ni-Fe oxide/(oxy)hydroxide layer with an optimal ratio of Ni/Fe elements, and the formation of the NiFe2O4 spinel phase on the surface of Ni-Fe/SS-NC.

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Figure 6. (a) OER polarization curves of Ni-Fe/SS-NC, Fe(Ni)/SS-PM, Ni-Fe/SS, and AISI 304 SS with j values normalized by ECSA, in 1 M KOH at a scan rate of 5 mV s−1 with iR compensation. (b) Comparison of normalized j and η values of the SS-based electrodes fabricated by different ways.

CONCLUSION In summary, a super-active and robust SS-based electrode for OER was fabricated by treatment of a cheap austenitic AISI 304 SS plate through a facile two-step process of acid corrosion and Ni (oxy)hydroxide deposition. This is an efficient approach to significantly enlarge the specific surface area of an SS substrate and to greatly improve the intrinsic activity of the SS-based electrode. Here the SS substrate serves not only as the high electroconductive current collector, but also as the robust nanostructure scaffold; in another aspect, the nanoparticulate Ni-Fe oxides/(oxy)hydroxides, especially the NiFe2O4 spinel phase, coating on the surface of NC arrays acts not only as active sites, but also as an effectively protective layer to prevent against inner corrosion of the SS substrate. The Ni-Fe/SS-NC electrode displayed high OER activity and good stability in 1 M KOH, thanks to the high conductivity and large ECSA of the self-supported SS NC array, the uniformly coated nanoparticulate Ni-Fe oxide/(oxy)hydroxide in the optimal Fe/Ni ratio, and last but not least, the NiFe2O4 spinel phase on the surface of 22


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Ni-Fe/SS-NC. The OER performance of Ni-Fe/SS-NC compares favorably with the best performance of electrocatalysts on planar substrates reported so far. In terms of high activity, good stability, simple work-up operation, low cost and easy scale-up fabrication process, as well as recyclable capability, the Ni-Fe/SS-NC electrode has bright prospect of large-scale application in a new generation of water splitting electrolyzers employing electric power from renewable energy sources.

ASSOCIATED CONTENT Supporting Information Additional experimental details, j‒t curves for electrode preparation processes, plots for ICP-OES analysis, additional SEM and TEM images, EDX histograms and elemental mappings, SAED and PXRD patterns, Raman spectra, XPSs, BET adsorption/desorption plots, polarization curves of Ni-Fe/SS-NC electrode, CVs of all as-prepared SS electrodes for measurement of the double layer capacitances, and current efficiency plots for calculation of Faradaic efficiency. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail for M.W.: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful to the National Natural Science Foundation of China (Nos. 21673028 and 21373040) and the Basic Research Program of China (No. 2014CB239402) for financial support

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of this work. The TEM images of this work were measured at the Pico Center at Southern University of Science and Technology (SUSTech) that receives support from Presidential fund and Development and Reform Commission of Shenzhen Municipality.

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Table of Content:

A Self-Supported Stainless Steel Nanocone Array Coated with a Layer of Ni-Fe Oxides/(Oxy)hydroxides as Highly Active and Robust Electrode for Water Oxidation Junyu Shen, Mei Wang,*, Liang Zhao, Jian Jiang, Hong Liu, and Jinxuan Liu

A self-supported stainless steel (SS) nanocone array coated with a layer of Ni-Fe oxides/(oxy)hydroxides displays high OER activity and good stability in 1 M KOH. Besides, it has merits of very low-cost, facile fabrication process, suitability for scale-up and recycling, which make it a promising electrode for application in commercial water-splitting electrolyzers.

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A Self-Supported Stainless Steel Nanocone Array Coated with a Layer

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